The principle governing the operation of the read sensor in a magnetic disk storage device is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). Magneto-resistance can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of what is termed a top spin valve are, starting at the lowest level, a free magnetic layer, a non-magnetic spacer layer, a magnetically pinned layer, and a topmost pinning layer. Inverted structures in which the free layer is at the top are also possible (and are termed bottom spin valves). Only the lowest layer of a bottom spin valve is seen in FIG. 1—antiferromagnetic layer 11.
Although the layers enumerated above are all that is needed to produce the GMR effect, additional problems remain. In particular, there are certain noise effects associated with such a structure. As first shown by Barkhausen in 1919, magnetization in a layer can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem has been to provide operating conditions conducive to single-domain films for MR sensor and to ensure that the domain configuration remains unperturbed after processing and fabrication steps as well as under normal operation. This is most commonly accomplished by giving the structure a permanent longitudinal bias provided, in this instance, by two opposing layer 16 which are separated by gap 13 (FIG. 1). Examples of hard bias materials include Cr/CoPt or Cr/CoCrPt (where Cr is 0–200 Å), CoPt or CoCrPt (100–500 Å). Also seen in FIG. 1 is capping layer of 17 of Ta or Ru with a thickness of 1–30 Å.
As track density requirements for disk drives have grown more aggressive, GMR devices have been pushed to narrower track widths to match the track pitch of the drive and to thinner free layers to maintain high output in spite of the reduction in track width. Narrower track widths degrade stability as the device aspect ratio starts suffering. Thinner free layers have traditionally degraded stability and increased the asymmetry distribution across the slider population. The thicker hard-bias that is typically used to overcome stability concerns associated with the junction also results in amplitude loss due to the field originating from the hard bias structure. Side reading, which is attributable to any deviation of the head microtrack profile from a square, also gets worse with narrower track widths
One approach that has been developed by the industry to overcome some of these stability concerns has been to use the lead overlay design shown in FIG. 1. In this design, track width is defined by the separation 18 of conductor leads 12 rather than by the hard bias separation 13. The lead overlay design moves the track edges, which are in part the cause of the instabilities, away from the current carrying region. Furthermore, the device has a more favorable aspect ratio, further enhancing stability. One remaining concern with such a device is whether or not it improves side reading. Although there is no substantial current in the area under the leads (overlap region), the region is still magnetically active and may transmit flux to the center of the device. The field due to the hard bias plugs gradually decays starting from the hard bias edge reaching a minimum at track center.
The two lines marked as 15a that extend under the leads a short distance from the bias plugs 16 represent the dead zone which is the magnetically inactive region between the wider physical width and the narrower magnetic width. Because of improper scaling (very high track density relative to linear density), the dead zone has become negative. i.e. the physical width has become narrower than the magnetic width.
A routine search of the prior art was performed with the following references of interest being found:                In U.S. Pat. No. 6,275,362, Pinarbasi shows a bottom SV process. In U.S. Pat. No. 6,292,335B1, Gill disclose a bottom SV process without a hard bias while in U.S. Pat. No. 6,222,707B1, Huai et al. reveal a related bottom SV process. U.S. Pat. No. 6,221,172B1 (Saito et al.) and U.S. Pat. No. 6,219,208B1 (Gill) are related SV MR patents.        